Second symptom:
Closing the GUI window does not always end the process (also experienced on a Windows host). CTRL-C has to be used.

Third symptom:
Trying to start a VM with 3D acceleration gets the error:

Failed to open a session for the virtual machine win10.
The VM session was aborted.
Result Code: NS_ERROR_FAILURE (0x80004005)
Component: SessionMachine
Interface: ISession {7844aa05-b02e-4cdd-a04f-ade4a762e6b7}

Thanks. /usr/lib64/dri/i965_dri.so is a regular file, is that correct? Is it also correct that this file has permissions 0777? In that case, that's the reason for your problem: The hardening code does not allow to load such libraries.

Seeing https://www.virtualbox.org/ticket/16693 it looks like VirtualBox refuses to load shared libraries with 777 permissions. On gentoo the dri libraries in /usr/lib64/dri are symlinks to libraries in /usr/lib64/mesa (thus 777) and VirtualBox is not smart enough to determine that they are symlinks and not actually world writable libraries, thus denying loading.